Field of the Invention
The invention concerns a method and magnetic resonance apparatus for reconstruction of a three-dimensional magnetic resonance image data set from magnetic resonance slice data of a target region acquired in target slices by operation of a magnetic resonance scanner of the apparatus while a noise object distorted the magnetic field in the target region, in particular a metal object.
Description of the Prior Art
Magnetic resonance tomography is an imaging modality in widespread use, with which it is possible to acquire magnetic resonance raw data from a three-dimensional target region inside a patient, by acquiring the data from multiple slices of the three-dimensional target region using 2D imaging for each slice. These different slices of the target region (target slices) normally form a so-called slice package which contains a three-dimensional raw data set, from which a three-dimensional magnetic resonance image data set is reconstructed.
It is known, for each target slice to be acquired, in addition to acquiring raw data from a central partition slice of the respective target slice in extent and location, to also acquire raw data from multiple partition slices adjacent to the central partition slice in a supplementary encoding direction perpendicular to the slice plane in a number of phase-encoding steps.
In order to acquire such magnetic resonance data of an object, for example of a patient, the object is normally introduced into a constant magnetic field (B0 field) of the magnetic resonance device, specifically into a homogeneity volume thereof in which only very slight deviations from the nominal value of the constant magnetic field are permitted. This causes nuclear spins in the target region to be oriented in the direction of the constant magnetic field. In order to acquire magnetic resonance raw data, the spins are excited (deflected from alignment with the constant magnetic field) by radio-frequency pulses (excitation pulses) generated by a radio-frequency coil arrangement magnetic resonance signals produced by the decay of this excitation are detected, and values of these signals are entered at respective data entry points in a memory. The acquired and entered raw data are collectively referred to as k-space. In order to enable a spatial encoding of the magnetic resonance data, rapidly switched magnetic gradient fields are superimposed on the constant magnetic field, in particular a slice selection gradient that restricts the excitation to one slice to be measured, a phase encoding gradient, and/or a readout gradient, which ultimately also defines the readout direction. The raw data present in k-space are converted by a Fourier transformation into the image domain in order to reconstruct a magnetic resonance image data set therefrom.
Reconstructing diagnostic-quality magnetic resonance images becomes problematic when the magnetic field to which the spins in the target region are subjected is distorted during the acquisition of the underlying raw data. One reason for such distortion is noise objects that influence the magnetic field in a distorting manner, in particular metal objects, for example implants or the like. Noise objects cause local distortion of the constant magnetic field, which means that exhibits a local inhomogeneity that influences the otherwise constant magnetic field both the excitation of the nuclear spins and the acquisition of the magnetic resonance signals.
An influence that is problematic when imaging different target slices occurs during the excitation. As mentioned, a slice selection gradient is normally activated that is intended to ensure that the magnetic resonance frequency used during the excitation is present only in the area of the slice to be excited, because the resonant frequencies of the nuclear spins in the other slices are shifted by the slice selection gradient such that an excitation of the spins in those other slices does not occur. This is based on the assumption that the constant magnetic field is homogeneous. If this assumption is not applicable, particularly a cuboid target slice will not be excited as is intended, but instead a distorted slice is excited, which can extend over a number of target slices. Acquired signals then do not originate from the desired target slice, but possibly from other target slices or even from outside the target region. This results in artifacts in the reconstructed image.
Magnetic resonance imaging of patients having metallic orthopedic implants is nevertheless becoming increasingly important, due to the rapidly growing population of persons with implants and the enhanced soft-tissue contrast of magnetic resonance imaging compared with other modalities, as well as due to improved magnetic resonance imaging methods that suppress the image distortions in the vicinity of noise objects in an increasingly better fashion than was previously the case.
A procedure for correction of said artifacts is known under the name of SEMAC and is described for example in an article by Lu et al., “SEMAC: Slice encoding for metal artifact correction in MRI”, Magnetic Resonance in Medicine 62, pp. 66-76 (2009) and in US 2010/0033179 A1 (where it is referred to under the name “SEPI-VAT”), wherein artifacts caused by metallic noise objects are corrected by a robust slice selection encoding of each excited slice in respect of metal-induced inhomogeneities. The general approach in this known procedure, as noted above, is to perform an additional phase-encoding operation perpendicular to the slice plane (in other words in the slice selection direction) for each target slice to be excited in order to be able to assign correctly in spatial terms the signals from other target slices occurring in the event of distorted excitation. The additional phase-encoding operation in the slice selection direction (supplementary encoding direction) therefore makes it possible to resolve the excitation profile, distorted on account of the noise object, of each target slice and to thereby avoid incorrectly sorting slice data assigned to the target slice, in other words the data are assigned to the correct target slice. In the aforementioned publications, this is performed on the basis of the VAT method. With regard to the procedure described therein, not only—as is already the case for VAT alone—are the “in-plane” distortions therefore reduced, but also distortions between the slices (“through-plane”) are reduced, because the acquired signals can be assigned to their actual physical target slices by Fourier transformations along the slice selection direction, namely the supplementary encoding direction. A problem in this procedure, however, is that the overall measurement time is increased significantly because of the multiplicity of additional phase-encoding steps required per target slice in order to resolve the respective excitation profile of each target slice.
Consequently, the spatial resolution in the slice selection direction (supplementary encoding direction) is chosen to be very low, for example in the range of 6-15 phase-encoding steps, in order to keep the measurement time to a minimum. This results in the further problem that the imaging capability of the Fourier transform into the spatial domain (image space) is extremely poor because of the lower resolution. The “point spread function” causes the acquired magnetic resonance signals to overshoot into adjacent target slices, which means that the image clarity is impaired, or in extreme cases incorrect image information is delivered. These more subtle limitations on the image quality are hardly noticeable in the distortion area, where the SEMAC correction is extremely effective, wherein image areas predominate that are sufficiently distant from the disturbances caused by the noise object and are thus not subject to any distortions, but where a reduction of the image quality may nevertheless occur as a result of undesired side-effects of the SEMAC reconstruction.